 Welcome to the sixth unit of NASA and the Sailor Foundation Space Systems Engineering online course, the trade studies. The unit objectives for Unit 6 are recognize that trade studies support decision making throughout the project lifecycle, explain the trade study process using a Mars mission example, and describe how a trade tree is used to organize interconnected trade studies. Subunit 6.1 introduction to trade studies. The system engineering method relies on making decisions all along the way through the design process by the use of trade studies, sometimes called trade off studies. These studies go through and allow you to make decisions whether it's early on in requirements development, about which requirement might be best to levy on your system, what level of performance might be best to levy within that requirement, all the way down to which one of these two different types of propulsion systems might be best for my mission. So trade studies will be used all the way through the process. There'll be comparisons made along the way and we'll talk more about this as to when you're doing a trade study, which choice, A or B or C, might have the better performance, might provide the better cost solution, might provide the better schedule, keeping you on schedule solution or the best risk posture. And depending on your design and how you're moving forward with the system, one of these might be more or less important to you and we'll talk about how you might use that in your evaluation. So in the end, trade studies will document a set of requirements, assumptions, and priorities that are used for the decision. A key part of kind of wrapping up a trade study is to make sure you've thoroughly documented why you made the decision. So that later if maybe you need to make a decision, you'll at least have everything documented so you can go through and better understand it. So trade studies will be used all throughout the development life cycle. And again, from an early standpoint, you could say that I might want to do trade studies to help resolve some of my requirements that I'm just trying to develop for a new system. So in some cases in the requirements unit we talked about that you might have a 2B determined or 2B resolved requirement where maybe the specification in a requirement maybe how heavy the spacecraft can be or some other performance aspect of it hasn't been well defined and so you might do some trade study to develop what should be the right number to put in there as that TBD or TBR resolution and so early on you'll be doing trade studies just in the help of the requirements process move along. Later as you get towards the back end of the cycle you'll still be doing trade studies looking at system design impacts at this point. So you might be looking at a couple alternatives for the propulsion system and want to know whether an electric propulsion system or a chemical propulsion system might be the best for your mission. So you do a much more detailed trade study you might get all the way down to where you're doing trade studies between different vendors who sell a thruster and maybe there's three or four available and you want to decide which one is the right one for your mission. So you might do a trade study at the core level to be able to make a decision as small as which thruster should I buy or what kind of gyroscope or some other component that you might want to buy. So another part and that's on the next slide here is that trade studies also support this whole make buy decision process because in some cases you're going to go off into the marketplace and say you know what I need to perform this one function in my spacecraft is pretty common and I can go off and buy it in the marketplace with a number of vendors and they sell space qualified units and I can integrate them into my system in other cases you're going to decide it's probably my requirements are too stringent or too different than anybody else's I need to go ahead and make that component myself and so you might do trade studies to help you determine is this a component of the spacecraft I should make myself or is this one that I should go off into the marketplace and buy. Another thing that trade studies are going to help you do alternative technologies there might be one or two different ways to accomplish some function on your spacecraft and maybe some of them use technologies that are available off the shelf or it's something available in the marketplace while maybe there's another technology that you would have to develop and it's a new technology and it's not currently available but that that new technology might be able to give you more better performance or more scientific return and so you're going to go through this assessment and a trade study of what's the risk of using the more or the less mature technology versus the risk of buying something that's already off the shelf and space qualified and you might look at the cost of those two and other design impacts they might have on your ability to meet the high level goals and objectives. So when you go about to perform a trade study what's most important is to define what are the things the criteria that you want to use to judge the options in front of you right so I want to know maybe I'm a mass constrained I don't want to build a very heavy spacecraft so how much each one weighs is going to be very important to me so maybe mass is always going to be a criteria whether I'm picking a small component of the spacecraft or a big component I always want to know which if I have options I want to know which one is the lighter of the two options and so there could be other times like if I'm building a vehicle that has to carry crew I'm really worried about the safety of the crew so if I have to make a decision about two competing parts of the spacecraft that can perform a function you know maybe two companies build a specific component for cleaning the air inside the cabin a filter system I'm going to compare them and say which one do I think is going to be safer for the crew to use during the time of the mission that they're going to be out there so these these become known as you see on this chart here a measure of effectiveness or a measure of performance and so the idea is that you're going to again write down what are these things that I want to use in evaluating options in a trade study to be able to help me make a decision and so that's that's what the key here is let's look through each one so a measure of effectiveness how effective is each option in meeting the requirements that you have for that system how well does it meet the requirements so you see a list here of a couple measures of effectiveness things like life cycle cost how much in the end would it cost me if I was to make decision to pick option A or to make a decision to pick option B how long will it take to complete the development if I pick option A or option B maybe one of them is something I can buy off the shelf one of them is going to need to be worked on and modified to meet my needs and so that could become a key this decider between the two things like crew safety again we talked about that a measure of effectiveness if I have two options and I want to use them in my spacecraft and it's a crew spacecraft which one do I think is safer to have in the crew cabin which one do I think is safer for the crew overall these are all measures that I might want to do these measures are going to be ways to evaluate each option through some analysis or study to be able to then compare them to see which one is the best for my application so the last one there is tolerance to faults right so if I buy this computer or that computer which one do I think is going to be more tolerant to faults back or the PC whatever you're going to use on your spacecraft right so might go through some evaluation to determine which one of those you think is going to be the most fault tolerant which might mean if it's more fault tolerant you can get away with not having too many backup systems but if it's a critical system fault tolerance may be key to you because you know that if that system was to fail it might take your whole mission down with it so that might be a really key criteria for you to use in your analysis so that's a measure of effectiveness so the other side is a measure of performance and in this world we're talking about much more quantitative measures of the performance of option A versus option B to help in making a selection so and for each area where you've defined a measure of effectiveness you might have a couple different measures of performance that kind of go along with it but again these are usually more discrete definable measures that you're going to be able to again compare options again so you see a list here of things like mass and power and thrust where you know if I have two options for computers and one's going to require a lot more electrical power to run then the other I might choose to go with the system that has a lower requirement for power if I'm flying a spacecraft that has a very limited amount of power available so again that's something you want to keep in mind is that sometimes you're going to be able to pick an option based on one criteria like power that might become the over operating decision and there might be a lot more subtle trade studies where many measures of performance will come in handy in your evaluation maybe how much mass, how much power how hot does it get so how much heat does it give off that might be important to your designers as well things like propellant how much propellant would I need to carry if I go with option A versus option B all of those can come in as measures of performance that you're going to use to evaluate this two options or many options I might have said two you could have option A, B, C and D that you all want to do your trade study to compare many options doesn't always have to be a head to head against two so but in both cases whether you're looking at a measure of effectiveness or measure of performance these are sometimes known as a figure of merit figure of merit meaning there's some way to gauge the merit of one option versus another or versus many options and so those figures of merit are things that are going to use to make sure that in the end you've kind of put together a good list of all the things that are important to you in decision making evaluated every option against that list of figures of merit and then presented that to the key decision maker whoever is making the decision so they could decide which one of the options to go with I think the most important evaluation criteria for a trade study are does it lead where you need to go does it solve the problem that you're going to eventually find with enough space for mistakes suppose you decide early on well we're going to use such a material like brilliant does it leave you margin for error what if the brilliant turns out to be harder to use than you think do you have a backup plan so do you have to build a special factory which we did so can you afford the result you have to be able to anticipate the steps that follow the decision that you're about to make so nothing is ever finished when you think it's finished you say okay we're going to use brilliant well then there's many many years in front of you before the brilliant is really ready to fly so there's no possible way to anticipate all of that so part of the decision process has to be is this a robust decision are you protected in case you don't understand everything is there any way to go back if you have to would those glass mirrors ever be able to meet the requirements if you had to use them so I think that's one of the most important criteria for a trade study is it really a final decision or not well the evaluation criteria for a trade really depends on the trade itself I don't think there are any really real fixed rules for this but certainly in our business you know spacecraft business mass and power are always going to be in there in the new way of doing business let's face it cost and schedule will always have to be in there as well so those four will certainly always have to be factors the other factors that you would pick would be whatever performance parameter is that issue that kind of thing so I think that you really have to look at each trade based on its own merit but those four will probably always have to be there decision analysis and chapter 12 of DOD's system engineering fundamentals